Anti-lock brake systems (ABS) are frequently included as standard equipment on modern vehicles and are currently required on almost all commercial vehicles. The operation of ABS is based on the principle that a skidding wheel (i.e., where the tire contact patch is sliding relative to the road) has less traction than a non-skidding wheel. Thus, by preventing a vehicle's wheels from skidding while the operator is attempting to reduce the speed of the vehicle, anti-lock brakes permit the vehicle to be stopped faster without compromising the operator's ability to steer or otherwise control the vehicle.
Anti-lock brake systems provide rapid, automatic cadence braking in response to signs of incipient wheel locking by alternately increasing and decreasing brake pressure in the brake line(s) of the affected wheel(s). ABS systems typically include speed sensors, a plurality of valves, and an electronic control unit (ECU). The speed sensors, which are located on each controlled wheel, provide the ECU with data indicating that one or more of the wheels are about to lock up. A valve is located in the brake line of each brake controlled by the ABS system for releasing brake pressure from the brakes.
Depending on the type of brake system employed, two different methods are used for recovering the brake pressure. In a hydraulically operated brake system, which is typically used in cars, a pump component is included in the system to restore hydraulic pressure to the brake line in which the pressure has been released by a valve. In a pneumatically operated brake system, which is typically used in commercial vehicles, compressed air is taken from an air reservoir that is fed from an air compressor.
The ECU is essentially a computer that monitors the speed sensors at all times and controls the valves. The ECU detects decelerations in the wheels that have fallen below a predefined deceleration range. When the ECU detects rapid wheel deceleration, it (i) reduces the brake pressure to one or more of the brakes until it detects acceleration, and (ii) increases the pressure until it senses deceleration again. The ABS system is capable of performing this function rapidly, before the wheel can lock. The result is that the speed of the ABS controlled wheels is always relatively close to the actual vehicle speed. This aspect of ABS provides optimal stopping and stability performance under dry or slippery road conditions.
Standard ABS systems are typically optimized for road (i.e., pavement) operation and allow for a minimal braking distance on roads while still providing sufficient cornering force. However, it has been recognized by those skilled in the art that under certain surface conditions anti-lock systems may actually require a longer braking distance than would be the case with continuously locked wheels, especially in the case of off-road surface conditions such as gravel, broken stone, crushed stone, loose ground, mud, high snow, etc. A possible explanation for this effect is that under such off-road conditions, a wedge of surface material is usually formed in front of the locked wheels (i.e., tires), thereby creating changes in the rolling friction value and slowing the vehicle down. Because the normal ABS mode utilizes small or shallow wheel speed cycles, the tire tends to overrun this wedge of material, whereas a locked wheel does not exhibit this tendency. Thus, in contrast to a vehicle's regular ABS mode, where wheel speed is controlled within an optimal slip range in the case of over-braking, the off-road mode includes higher slip rates for deeper wheel cycles with temporary wheel lock-ups that are enforced for the benefit of a shorter stopping distance. However, the disadvantage of an ABS controlled wheel in the off-road mode is reduced or detracted side force and reduced vehicle stability and tractability. Thus, despite the benefit of a shortened braking distance, the result is not necessarily desirable because the vehicle cannot be adequately controlled while its brakes are in a locked state.
Federal performance and equipment requirements for braking systems on vehicles equipped with air brake systems have been established and can be found in Federal Motor Vehicle Safety Standard (FMVSS) 121. FMVSS 121 imposes requirements for stopping distance and curve stability which must be complied with even when a vehicle's ABS system is operating in the off-road mode.
The FMVSS 121 curve stability test requires that a vehicle that drives and brakes within a 500-foot (152 m) radius-curve stay within the 12-foot (3.66 m) range of a slippery curve lane. The initial speed when the vehicle starts braking must be at least 75% of the maximum drive through speed without braking. The adhesion-slip curve between the tires and the road surface dictates that the transferable side force is increasingly affected as the wheel slip increases. The wheel slip is the actual wheel speed in percent below the actual vehicle speed. A non-braking, and therefore free-rolling wheel, has 0% slip, which means vehicle and wheel have the same speed. An over-braked wheel that has reached the slip rate of 100% is completely locked, resulting in reduced transferable side forces. Thus, there is a need for a method for controlling ABS systems in off-road conditions that addresses and overcomes these difficulties.